Needs for high performance plastics have been recently raised more and more and many polymers having various new characteristics have been developed and are available in the market. Among them, optically anisotropic liquid crystalline polymers characterized by parallel arrangement of their molecular chains are given attention because they have excellent mechanical properties.
As one of these liquid crystalline polymers, thermotropic polyesters are well known and, for example, p-hydroxybenzoic acid homopolymer and its copolymer copolymerized with 4,4'-dihydroxybiphenyl and phthalic acid are well known. However, these homo- and co-polymers of p-hydroxybenzoic acid are inferior in melt-fluidity as their melting points are too high. Therefore, polyesters whose melting points are lowered by copolymerizing various components with p-hydroxybenzoic acid have been applied for to obtain patents. For example, a polyester prepared by copolymerizing phenylhydroquinone, terephthalic acid and/or 2,6-naphthalenedicarboxylic acid with p-hydroxybenzoic acid (Japanese Patent Publication No. 500,215/1980), a polyester prepared by copolymerizing 2,6-dihydroxynaphthalene and terephthalic acid with p-hydroxybenzoic acid (Japanese Patent Laid-Open No. 50594/1979), a polyester prepared by copolymerizing 2,7-dihydroxynaphthalene, isophthalic acid and/or terephthalic acid with p-hydroxybenzoic acid (Japanese Patent Laid-Open No. 38426/1985, No. 104,123/1985), a polyester prepared by copolymerizing hydroquinone, isophthalic acid and moreover a small amount of 4,4'-dihydroxybiphenyl and terephthalic acid with p-hydroxybenzoic acid (Japanese Patent Laid-Open No. 38425/1985, No. 235,833/1985), a polyester consisting of p-hydroxybenzoic acid, 4,4'-dihydroxybiphenyl or hydroquinone, terephthalic acid and isophthalic acid (Japanese Patent Publication No. 47870/1972), a polyester prepared by copolymerizing phenylhydroquinone, 4,4'-dihydroxybiphenyl and terephthalic acid with p-hydroxybenzoic acid of less than 57 mole % of the whole (Japanese Patent Laid-Open 199,028/1985) and a polyester prepared by copolymerizing p-oxybenzoic acid, an aromatic dihydroxy compound and terephthalic acid with polyethylene terephthalate (Japanese Patent Laid-Open No. 8395/1976) are disclosed.
Therefore, the present inventors carried out polymerization, molding and evaluation of various new polyesters to obtain polyesters capable of providing moldings having good flow characteristics, heat resistance and mechanical properties and found that aromatic polyesters consisting of specified amounts of p-hydroxybenzoic acid, 4,4'-dihydroxybiphenyl, hydroquinone and terephthalic acid or terephthalic acid and isophthalic acid as well as aromatic polyesters prepared by copolymerizing dihydroxy compounds such as t-butylhydroquinone and so on with these polyesters could satisfy the above described purpose.